Method of forming fluorescent screens

ABSTRACT

A method of forming a fluorescent screen on the surface of a substrate, comprising the steps of: applying a layer of a fluorescent substance and a resin binder onto one side of a film sheet which is soluble in a first solvent but insoluble in a second solvent in which said binder is soluble so as to form a fluorescent film assembly, coating a resin binder onto the substrate surface and contacting said one side of said fluorescent film assembly thereto to cause it to adhere to the substrate and bonding said one side of said film sheet to said substrate surface by means of said resin binder which is soluble also in said second solvent so that said layer of fluorescent substance on said fluorescent film assembly adheres to said substrate surface, and, then, removing said film sheet by dissolving it in said first solvent.

United States Patent Kubota [54] METHOD OF FORMING FLUORESCENT SCREENS [72] Inventor:

[73] Assignee:

Tsutomu Kubota, Tokyo, Japan Tokyo Shibaura Electric Co., Ltd., Kawasaki-511i, Japan 22 Filed: Dec.l1,1969 211 Appl.No.: 880,497

Related US. Application Data [63] Continuation of Ser. No. 451,479, Apr. 28, 1965,

abandoned.

[30] Foreign Application Priority Data Apr. 30, 1964 Japan ..39/24243 [51] Int. Cl.

[56] References Cited UNITED STATES PATENTS 2,588,569 3/1952 Picard..... ....96/36.1 UX 2,827,390 3/1958 Garrigus.. ....96/36.1 UX 2,967,783 l/l961 Shinal ..96/36.l X

Prepare a slurry by mixing a water soluble binder and a fluorescent substance Apply the slurry to a film and dry Bend the film too substrate surtoce 5] Mar. 14, 1972 2,959,483 11/1960 Kaplan ..96/36.1X

OTHER PUBLICATIONS Freedman et al., RCA Review, Vol. 12, Sept. 1951, p. 568 and pp. 577- 578 relied on.

Leoy et 21., The Prep. of Phosphor Screens for Color Television Tubes, Apr. 12- 16, 1953, pp. 1- 3 relied on.

Sodowski et al., Photodeposition of Luminescent Screens, JOES, Vol. 105, No. 2, Feb. 1958, pp. 105- 107 relied on.

Primary Examiner-Charles L. Bowers, Jr. A tt0rn e y George B. Oujevolk [57] ABSTRACT A method of forming a fluorescent screen on the surface of a substrate, comprising the steps of: applying a layer of a fluorescent substance and a resin binder onto one side of a film sheet which is soluble in a first solvent but insoluble in a second solvent in which said binder is soluble so as to form a fluorescent film assembly, coating a resin binder onto the substrate surface and contacting said one side of said fluorescent film assembly thereto to cause it to adhere to the substrate and bonding said one side of said film sheet to said substrate surface by means of said resin binder which is soluble also in said second solvent so that said layer of fluorescent substance on said fluorescent film assembly adheres to said substrate surface, and, then, removing said film sheet by dissolving it in said first solvent.

4Claims,3DrawlngFigures dissolving to transfer the fluorescent Remove the film from the substrate surface by substance onto the substrate surface PATENTEDMAR l4 I972 3.649.269

Prepare a slurry by mixing a water soluble binder and a fluorescent substance Apply the slurry to a film and dry Bond the film toa substrate surface Remove the film from the substrate surface by dissolving to transfer the fluorescent substance onto the substrate surface FIG.I

METHOD OF FORMING FLUORESCENT SCREENS This application is a continuation of application Ser. No. 451,479, now abandoned, filed Apr. 28, 1965.

This invention relates to a method of forming a fluorescent film on a substrate surface such as the surface of the face plate of a cathode ray tube, for example, and more particularly to a method of forming not only a conventional homogeneous fluorescent screen but also fluorescent films of desired configuration or pattern on a substrate surface.

Many methods have been used to form fluorescent films on the surface of a certain kind of substrate. Typical examples of these prior methods include a method wherein a fluorescent substance is dispersed in a vehicle, and the vehicle is applied onto a substrate surface by brushing or spraying to form a fluorescent film; a method wherein a suspension of a fluorescent substance is caused to flow down along the inner surface of a tube similar to the method of preparing the fluorescent surface of a fluorescent lamp; a method wherein a sulfur vapor is passed through a cathode ray tube to cause sulfur to deposit on the wall thereof and then a powder of fluorescent substance is introduced in the tube to be deposited on the surface of the tube as in manufacturing of the fluorescent screen of a cathode ray tube; a method wherein an alcoholic suspension of a fluorescent substance is sprayed onto the surface of a substrate to form a fluorescent film; a method wherein a fluorescent substance suspended in water glass is caused to flow along a substrate surface so as to precipitate the fluorescent substance thereon; a method wherein a fluorescent screen is provided by printing a fluorescent substance on a substrate surface by the silk screen technique, for example; and a method comprising the steps of flowing a slurry of light-sensitive fluorescent substance on a substrate surface, rotating the substrate to spread the slurry over the entire surface so as to dry the slurry, then irradiating the slurry with ultraviolet ray to harden a binder, and developing the film to remove undesired portions which were not subjected to ultraviolet irradiation so as to form a desired fluorescent film. Of these prior art methods, with regard to the particular method of using a brush, it is difficult to provide a fluorescent film of uniform thickness by this method. This is especially true in a curved surface. While spraying can provide coated films of relatively uniform thickness, loss of paint is excessive. Where a slurry containing a fluorescent substance is made to flow on the substrate surface it is necessary to main- I tain a uniform thickness of the slurry such as by imparting a suitable movement to the substrate until the slurry dries. This method is not applicable to a substrate surface of irregular configuration. A method wherein a fluorescent film is formed by applying a fluorescent substance suspended in water glass onto a substrate is limited to a substrate provided for a vessel that can contain water and hence can not be applied to substrates having irregular surfaces. Moreover by the prior art methods, in order to form a fluorescent screen of the desired characteristics, for example a screen that can emit light rays of different colors from different stripes on dots it is necessary to have independent coating, exposure and development operations for different stripes, dots and colors, thus complicating the manufacturing procedures extremely. Especially, when the light-sensitive substance is utilized as a binder, the brightness of the fluorescent screen produced is decreased.

An object of this invention is to provide a novel method capable of fonning a fluorescent film of uniform thickness, of the desired configuration and pattern on a flat plate as well as a curved surface of a substrate.

Another object of this invention is to provide a method of applying a fluorescent substance onto a substrate surface without appreciable loss of the material.

Still another object of the invention is to provide a method of forming a fluorescent film without decreasing its brightness.

Yet another object of this invention is to provide a method of forming a fluorescent film of any desired pattern.

Briefly stated, this invention contemplates the steps of forming a layer of a fluorescent substance of a first color on a film which is soluble in a first solvent for example an organic solvent, but insoluble in a second solvent, for example water. The fluorescent substance on the otherhand is insoluble in the first solvent but soluble in the second solvent. The film is made to adhere onto a substrate surface by means of a binder which is soluble in said second solvent. Thereafter said film is removed by dissolving it by means of said first solvent so to form a fluorescent film on the substrate.

Those features of the invention which are believed to be novel are specifically set forth in the claims attached hereto. The invention will, however, be better understood and further advantages thereof appreciated from a consideration of the following description and drawings, in which:

FIG. 1 is a block diagram showing successive steps of forming a fluorescent film in accordance with one example of this invention; and

FIGS. 2a and 2b are sectional views showing different steps of said example.

Referring now to the accompanying drawing, a suspension (in the form of a paste or a slurry) is prepared by incorporating a fluorescent substance into a water soluble binder. The suspension is then applied to a desired thickness onto a sheet or film ll of plastic, for instance, which is soluble in organic solvents but insoluble in water, by means of a film-coating machine provided with a doctor knife or an air knife and then dried to form a fluorescent film or layer 10. The layer may be applied on the entire surface of the film 11 or a portion thereof, the latter mode of coating being utilized to form a figure or pattern. Such a partial coating may be provided by printing a portion of the plastic film or by coating the entire surface thereof and then removing undesired portions by mechanical working or by applying a mixture of a fluorescent substance and a light sensitive substance and then removing undesired portions by a photochemical technique.

Then the coated film 11 is placed on one surface of a substrate 13 such as a face plate of a cathode ray tube by means of a water-soluble binder 12 with the fluorescent film 10 facing the substrate surface. After setting the binder, the film 11 is dissolved off by a solvent capable of dissolving it to transfer the film 10 onto the substrate surface so as to form the desired fluorescent film 110, as shown in FIG. 2b.

The configuration of the substrate utilized in this invention may be any suitable shape. For example, where the surface of the substrate is not flat the base film may be molded beforehand by using a thermoplastic material to have a surface corresponding to the surface of the substrate and then cemented to it.

The following specific examples are given by way of illustration and are not to be construed as limiting in any way the scope and spirit of the invention.

EXAMPLE 1 A slurry comprising 10g. of polyvinyl alcohol, 50g. of fluorescent substance consisting of zinc sulfide and cc. of water was applied on a polystyrene paper of 0.50 mm. thick to a thickness of about 0.02 mm. by a conventional method and then dried to form a fluorescent film assembly. A convex substrate made of polyester was used having the same surface as the internal surface of a cathode ray tube, including a face plate whose diagonal dimension was 14 inches. The base film was molded at a temperature of about C. to the screen on 'the side of the base film which did not engage said convex substrate. Thereafter the base film was cut to have a configuration and dimensions corresponding to those of said cathode ray tube internal surface. A solution of polyvinyl alcohol was then applied by spraying on the face plate of the cathode ray tube and said fluorescent film assembly was caused to adhere with its fluorescent screen in contact with the inner surface of the cathode ray tube. The assembly was dried at a temperature of below 60 C. After the entire surface of the fluorescent film was in close adherence to the internal surface of the cathode ray tube the interior of the tube is treated with 100 cc. of benzene to completely dissolve the polystyrene paper of the fluorescent film assembly. After removing the solution, 100

cc. of benzene was again introduced in the cathode ray tube to wash the internal surface. When dried, polystyrene contained in the benzene remained in the tube to form a thin film on the internal surface of the cathode ray tube. This film served as the base film for a vapor-deposited aluminum coating.

EXAMPLE 2 A suspension was prepared consisting of 8 g. of polyvinyl alcohol, 6 g. of zinc sulfide and 100 cc. of water. An enclosure of 10 cm. l cm. was formed on a flat polystyrene paper mounted on a glass sheet and the suspension was poured into the enclosure to fill it. The suspension was then dried while the polystyrene paper was maintained in the horizontal position to form a fluorescent film assembly, including a film of zinc sulfide and polyvinyl alcohol adhering to the polystyrene paper. A thin film of water glass was coated on a separate glass sheet and the fluorescent film assembly was caused to adhere to said water glass film by contacting the film consisting of zinc sulfide and polyvinyl alcohol therewith. After drying, the polystyrene paper was dissolved by means of trichlene. Thereafter, the glass sheet was fired at a temperature of 350 C. to oxidize and evaporate polyvinyl alcohol to obtain a glass sheet having a fluorescent film of cm. l0 cm.

In this example, when the polystyrene paper with a thin fluorescent film of zinc sulfide and polyvinyl is so worked as to obtain a desired figure or pattern, a fluorescent film of the desired figure or pattern can be provided on the glass sheet.

instead of mechanically working the polystyrene paper as above described for the purpose of providing a fluorescent film of the desired figure or pattern, a light-sensitive aqueous solution comprising 3 g. of ammonium bichromate and 100 cc. of water may applied by brush on a zinc sulfide fluorescent substance-polyvinyl alcohol film formed on the polyethylene paper and then dried. After putting the film with the desired figure onto the dried film, the composite film was then irradiating by a 100 v., 30 A. carbon arc lamp situated at a distance of 50 cm. Thereafter, the film was developed with water to set portions, of the polyvinyl alcohol irradiated by light rays while dissolving in water the remaining portions so as to form a fluorescent film of the desired figure or pattern on the polystyrene paper. This fluorescent film was then treated in the same manner as described hereinabove to provide on the glass sheet a fluorescent film of the desired figure or pattern.

EXAMPLE 3 A polystyrene paper of 0.25 mm. 35 cm.X30 cm. was placed on a flat aluminum sheet of 30 cm.X25 cm. and the peripheral edges of the polystyrene paper were folded back on the aluminum sheet and fastened thereto by an adhesive tape. in this case care was taken to closely contact the polystyrene paper with the flat aluminum sheet. The aluminum sheet was placed on the bottom of a glass vessel of the dimension 35X30X20 cm. and having a water drain port at the bottom. After closing the water drain port 50 cc. of 5 percent aqueous solution of barium nitride was added to the vessel and thereafter L. of deionized water was added. A suspension consisting of 5.5 g. of blue fluorescent substance of zinc sulfide, 30 cc. of 15 percent aqueous solution of water glass and 300 cc. of deionized water was poured into the cushion liquid prepared as above described by means of a sprinkling funnel. The mixture was left standing still for minutes to precipitate the zinc sulfide fluorescent substance and then the bottom drain port was opened to drain the remaining liquid. Thereafter the aluminum sheet bonded with the polystyrene paper was removed from the vessel and dried in a stream of drying air. Subsequent to drying the aluminum sheet was mounted in a whirler with the surface of the fluorescent substance directed upward and a light sensitive liquid consisting of 5 g. of polyvinyl alcohol, 100 cc. of deionized water and 0.5

g. of ammonium bichromate was poured onto the aluminum sheet to uniformly cover the fluorescent screen and the aluminum sheet was then dried at a temperature of less than 50 C. A separately prepared negative film including stripes of black lines of 0.75 mm. width each and white lines of 0.3 mm.

width each was placed on the fluorescent screen and the assembly was put in a printing frame to be irradiated for 3 minutes by a I00 v. 30-A. carbon arc lamp positioned at a distance of 50 cm. The aluminum sheet was removed from the printing frame, immersed in water for 1 minute and then developed by subjecting it to water spray to produce strips of the fluorescent substance of 0.3 mm. wide and spacings 0.75 mm. Similar procedures were made for green and red fluorescent substances. As this time portions of the stripes of blue fluorescent substance were covered by a cellulose tape to prevent adherance of the subsequently applied fluorescent substances. When exposing to light rays the white lines of the negative films were positioned close to and in parallel with the stripes by utilizing the stripes as the reference. Thus it is possible to prepare a polystyrene paper having parallel stripes of blue, green and red fluorescent substances. This polystyrene paper having stripes of the fluorescent substances was then bonded to the internal surface of the face plate of a cathode ray tube including a flat substrate surface by means of a thin film of a 1 percent aqueous solution of polyvinyl alcohol applied to said inner surface with the fluorescent film in contact with the inner surface of the face plate. At this time, care should be taken not to include air bubbles. After standing still for 2 hours benzene was poured onto the face plate to dissolve off said polystyrene paper and then benzene was removed. After washing the surface of the fluorescent substance with fresh benzene the assembly was dried to form a thin film from a small amount of polystyrene contained in the benzene remaining on the fluorescent substances so as to provide a face plate for a cathode ray tube having a striped screen of three colored fluorescent substances of uniform film thickness.

EXAMPLE 4 A polystyrene paper of 40 cm. diameter and 0.5 mm. thick was put on a Bakelite lamination of the same diameter and to mm. thick, and a Bakelite cylinder of 40 cm. outside diameter, 10 mm. wall thickness and 50 mm. height was put thereon in concentric relation. The bottom of the Bakelite cylinder was fastened to the Bakelite lamination by means of four clips. A rubber packing was provided at the lower surface of the Bakelite cylinder to form a shallow bucket with a bottom of polystyrene paper. This bucket was mounted on a slurry machine utilized in preparing shadow mask color cathode ray tubes. After wetting the polystyrene paper with an aqueous solution of a suitable surface active agent (Dryzell diluted with 200 parts of water) a slurry of green fluorescent substance was applied by a process commonly utilized for applying slurries. The composition of this slurry comprised 30 g. of a green fluorescent substance, 100 cc. of a 10 percent aqueous solution of polyvinyl alcohol, cc. of ion exchange water and 5 cc. of a 20 percent aqueous solution of ammonium bichromate. While rotating the bucket said slurry was poured onto the polystyrene paper to spread it over the entire surface of the paper and while heating and drying from above at a temperature of less than 50 C. the bucket was inclined to remove surplus slurry from the bottom portion thereof so as to form a fluorescent film of uniform thickness on the polystyrene paper. Thereafter the polystyrene paper formed with the fluorescent film was removed from the bucket and then irradiated and developed in the same manner as described in connection with Example 3 by utilizing a negative film having a stripe pattern consisting of white lines of 0.2 mm. width each and black lines of 0.3 mm. width each so as to provide stripes of blue fluorescent substance of 0.3 mm. width. Then portions of the stripes were covered by a cellulose tape and a blue fluorescent substance was applied thereon by the procedure described above. A negative film was used to effect similar printing and development by utilizing as the reference, stripes of the green fluorescent substance remaining outside of what was coated by the blue fluorescent substance owing to the presence of a cellulose tape so as to cause the blue fluorescent substance to remain so as to fill alternate spaces between stripes of green fluorescent substance. By the same process as above described, red fluorescent substance was applied on the remaining spaces between stripes of green fluorescent substance so as to obtain a polystyrene paper with stripes of fluorescent substances arranged in the order of green-bluegreen-red-green-blue-green-red. 0.5 percent aqueous solution of polyvinyl alcohol was applied on the inside of a cathode ray tube having a cylindrical screen surface of type to form a thin film and said polystyrene paper which has been cut to the same size of the screen surface with due consideration of the position of stripes of fluorescent substances was bonded to the face plate with the fluorescent film in contact with the inner surface of the face plate. After standing still for one night toluene was poured on said face plate to dissolve the polystyrene film and by removing most of the remaining polystyrene by additional toluene a fluorescent film protected by a very thin film of polystyrene was resulted.

EXAMPLE 5 Instead of utilizing a slurry having incorporated therein a light-sensitive substance as in Example 4, a fluorescent film of any desired figure may be produced by spraying polyvinyl alcohol or other suitable binder on a polystyrene paper so as to apply a fluorescent substance thereon, forming a film of a light-sensitive resin on the fluorescent substance, producing a fluorescent filrn of a desired pattern by utilizing negative film having said pattern, and bonding the fluorescent film to the substrate surface by a method analogous to that described above. Alternatively, similar fluorescent film may be produced by applying a fluorescent substance on a polystyrene paper by the aid of polyvinyl binder, imparting light-sensitive characteristic to the fluorescent film by means of a solution of ammonium bichromate and then treating the film in the same manner as above described.

Although in the above-described examples, a light-sensitive fluorescent film was formed on a polystyrene paper, then the film was exposed to light and developed and the resultant film of a desired pattern was bonded to the surface of a substrate, it should be understood that this invention is not limited in any way to such particular process steps. For example, afier bonding a film of a fluorescent substance to a substrate surface a fluorescent film of the desired figure or pattern can be produced by utilizing a negative mask.

Printing techniques can also be used, for example, fluorescent substances for emitting desired primary colors are added to an aqueous solution containing one or more watersoluble binders such as polyvinyl alcohol, polyacrylamide, carboxylmethyl cellulose, et cetra so as to prepare several types of printing ink. Thereafter these inks are used to print successive stripes on a film such as a polystyrene paper, cellulose acetate and the like which are soluble in benzene, toluene, xylene or mixed organic solvent consisting of one or more of these organic solvents and ketone or alcohol but not insoluble in water. The printing technique used can be a silk screen or intaglioprinting technique which can apply inks in a relatively large thickness. After drying the fluorescent inks printed on the film, thus providing a fluorescent screen, the film is bonded to the desired substrate surface within a cathode ray tube by using an aqueous solution of polyvinyl alcohol, polyacrylic amide, carboxylmethyl cellulose or water glass with printed side thereof contacted with the substrate surface. Thereafter, the substrate film is dissolved by said organic solvent so as to form a fluorescent screen.

The thickness of the substrate film soluble in organic solvents utilized in this example is not limited to any particular value but preferable thickness is in a range of from 0.1 to 0.2 mm. when easiness of handling and dissolving and mechanical strength are considered. Formed polystyrene sheet is suitable because of its good solubility. Where the substrate film is made of thermoplastic material such as polystyrene or cellulose acetate, it is possible to readily form fluorescent films on curved surfaces because such a substrate can be deformed under heat to align with the curved surface after it has been printed with an ink containing fluorescent substance.

While the concentration of an aqueous solution of the water soluble binder utilized as the printing ink is not limited by the quantity of incorporation of the fluorescent substance, where the ink is prepared from 2 parts of the fluorescent substance and 1 part of said aqueous solution concentration of 7 to 25 percent by weight, results in inks of suitable viscosity and easy to use. Water glass is not suitable because an ink utilizing it requires relatively long drying time after printing. Also the concentration of the binder utilized to bond said printed and shaped fluorescent layer to the surface of a substrate is not limited to any particular value but 10 percent, by weight, was found suitable.

In this embodiment the principle of transfer picture is utilized. More particularly, fluorescent films of the desired shape and pattern are sequentially printed on a sheet or film of substrate which is readily soluble in organic solvents by utilizing inks containing fluorescent substances of different primary colors, and then the printed film of substrate (if required after being shaped) is bonded to the desired substrate surface of the cathode ray tube. Instead of tearing off the bonded substrate film, it is dissolved off means of organic solvents so as to form the desired fluorescent screen. Although in the prior method of preparing screen for use in chromatron-type color-receiving tubes, fluorescent films of different shape, stripes pattern and color were successively formed on the surface of a substrate, this invention can not only eliminate such a troublesome procedure but also can be practiced at room temperature and without utilizing any light source. Even when the surface of the substrate which constitutes the fluorescent screen is curved as in conventional cathode ray tubes, it is possible to closely bond the fluorescent film to such a curved surface by printing the fluorescent film on a sheet of a thermoplastic material and then premolding this thermoplastic sheet under heat to the same contour as the substrates surface.

Moreover, as the binder utilized as the ink and for bonding the fluorescent film to the substrate is soluble in water it would not affect these films and substrate. As a result the printed and molded layer of the fluorescent substance would not be deformed or damaged because of the absence of elongation and extension of the substrate film. Further the substrate film can be readily removed by dissolving it in an organic solvent after the fluorescent film has been bonded thereto. As a result of experiment I have found that a polystyrene substrate having a thickness of 0.2 mm. and adapted to form the fluorescent screen of a 16 inch cathode ray tube could be dissolved in less than 1 minute by benzene and was so perfectly removed that there was no impurity remaining on the fluorescent screen prepared according to the method of this invention.

As can be noted from the foregoing description according to the method of this example fluorescent screens of any desired shapes and patterns can be very simply and readily formed on substrates having flat or curved surfaces, and as the fluorescent screens thus formed are free from any light sensitive substance there is no fear of decreasing their brightness.

EXAMPLE 6 It was found that when a water-soluble binder is used not only to bond a plastic film provided with the desired fluorescent substance to the substrate but also as an ink for printing the fluorescent substance, this substance tends to blur. By this reason it is necessary to convert the binder mixed with the fluorescent substance to a state insoluble in water after the fluorescent substance has been bonded to the substrate. This example shows use of a binder which can be convertible to water-insoluble state when irradiated by light rays or other radiations.

More particularly, as the fluorescent substance, zinc sulfide was used; and as the binder convertible to water insoluble state upon being irradiated by radiations, polyvinyl alcohol containing ammonium bichromate was used. The composition of the fluorescent ink was as follows 20 g. l g. 200 g.

water 100 cc.

The fluorescent ink prepared by thoroughly mixing this composition was applied by printing on a foamed polystyrene paper of 0.2 mm. thick X 450 mm. X 500 mm. As the polystyrene paper is insoluble in water and not swollen by water it would not undergo any deformation when printed with this fluorescent ink. Then the printed surface was irradiated for minutes by ultraviolet rays which emanated from a A. carbon are situated at a distance of 1.5 meter. When this radiation source is used, irradiation for 5 to 10 minutes from a distance of l to 1.5 meter is generally satisfactory. As a result of this irradiation, polyvinyl alcohol contained in the fluorescent ink will set to a water-insoluble state. Then the polysytrene paper having a printing of fluorescent film was molded in a mold having the same curved surface as the inside substrate of the cathode ray tube. A 1.0 percent aqueous solution of polyvinyl alcohol serving as a binder was applied on the surface of a substrate contained in a cathode ray tube, and the film printed with the fluorescent substance was urged against the substrate by means of a resilient body such as a soft polyurethane foam with the printed surface in contact with the applied binder.

In this state as the fluorescent ink has been converted to water-insoluble state there was no fear of disintegrating the printed pattern when contacted with the water contained in the binder. After drying the assembly at room temperature to solidify the binder, benzene was poured onto the assembly to dissolve the polystyrene paper. After removing the benzene solution the assembly was dried to remove nearly all of polystyrene. Since polyvinyl alcohol is not soluble in water the desired printed pattern of the fluorescent substance remained on the substrate surface.

When a relatively small quantity of benzene is used a portion of polystyrene will be left as a thin film covering the printed fluorescent film after benzene has been evaporated, so that is is possible to form a so-called metal back by depositing thereon aluminum by vacuum deposition.

EXAMPLE '7 While in Example 6 polyvinyl alcohol containing ammonium bichromate was used as a binder for the fluorescent substance that can be hardened by irradiation, such a binder is disadvantageous in that it decrease the brightness of the fluorescent substance.

This example is directed to obviate such difficulty. More particularly, a fluorescent film was prepared on a water insoluble film by printing or coating a fluorescent ink utilizing polyvinyl alcohol as the binder and a portion of the polyvinyl alcohol was converted to formal to render it water resistant. Thus the formalized polyvinyl alcohol is effective to firmly secure the fluorescent film when it is bonded by an aqueous binder so that the fluorescent film will not blur when it is transferred onto the surface of a substrate of a cathode ray tube by using an aqueous binder, thus forming sharply defined fluorescent screen.

For example, a fluorescent slurry consisting of 60 g. of polyvinyl alcohol, 500 cc. of water and l80 g. of zinc sulfide fluorescent substance was applied on a polystyrene paper to a thickness of 0.25 mm. by means of a dip coating machine, and then dried. When this polystyrene paper provided with a uniform coating of the fluorescent slurry was dipped in water it was noted that a portion of white powders of the fluorescent substance was released in water within less than 30 seconds. On the other hand said polystyrene paper was immersed for minutes in a formalizing bath consisting of 100 g. ammonium sulfate, 20 g. of 37 percent formaline, 80 g. of nitric acid (d 1.84) and 200 cc. of distilled water to formalize a portion of the fluorescent film applied on said polyethylene paper. The paper was washed with water followed by drying. Even when the partially formalized fluorescent film was immersed in water for more than l0 minutes it was found that powders of the fluorescent substance were not released in water. A

separately prepared glass substrate was covered by pouring on it a 2 percent aqueous solution of polyvinyl alcohol and after removing excess aqueous solution by inclining the glass sheet said polystyrene paper was bonded to it with said partially formalized fluorescent film in contact with the binder. During this transfer printing no spot was formed in the fluorescent film. The glass sheet bonded with the paper was left at a standstill for one night and then dipped in a bath filled with toluene to dissolve polystyrene paper. The glass was taken out of the bath and then washed with fresh toluene to remove most of the remaining polystyrene. After drying, fluorescent film transferred on the glass sheet was obtained. When the quantity of toluene is reduced to a proper value a small amount of polyethylene will remain on the sheet so that a thin film of polyethylene is for providing a metal back may be produced on the fluorescent film after evaporation of toluene.

As a further example a fluorescent ink consisting of 20 cc. of 10 percent aqueous solution of polyvinyl alcohol and 200 g. of zinc sulfide fluorescent substance was printed on a polystyrene paper of 0.25 mm. thick by using a silk screen printing technique and the printed paper was dried for one night at room temperature. Thereafter the paper was immersed for 10 minutes in a liquid consisting of g. of ammonium sulfate, 28 g. of 27 percent formaline, 70 g. of sulfuric acid (d 1.84) and 200 cc. of distilled water, and then dried. When the polystyrene paper not subjected to this treatment was immersed in water it was noted that a portion of the fluorescent substance had dissolved in water in less than approximately 20 seconds, but the polystyrene paper treated as above described did not show any release of the fluorescent substance even when it was immersed in water for more than 10 minutes. A binder consisting of 5 cc. of 10 percent solution of polyvinyl alcohol and 55 cc. of water was applied to coat the entire surface of a face plate of a cathode ray tube said fluorescent film which has been undergone said formalizing treatment was bonded under pressure to the face plate. Since the fluorescent film transfer printed in this manner has no tendency of dissolving and disintegration by the water soluble binder it can preserve its brightness during the life of the cathode ray tube.

Thus in accordance with the method of this invention, layers of fluorescent substances of the desired color and pattern are applied on a film to a desired thickness then these layers are bonded to the surface of a substrate to which the layers are finally to be transferred, and then the layer is dissolved away to leave only the fluorescent layers. Moreover as the film is flexible, and can be shaped into a curved surface it is able to readily form layers of fluorescent substance of the desired shape, pattern and uniform thickness not only on flat plate but also on curved plate. Further as the layers of fluorescent substance are placed on the film by printing or other method it is not only possible to avoid undesired loss of the fluorescent substance but also to readily form desired patterns, so that the invention is particularly suitable for providing fluorescent screens in color cathode ray tubes.

While the invention has been explained by describing particular embodiments thereof, it will be apparent that improvements and modifications may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. A method of forming a fluorescent screen on the surface of a substrate, which method comprises:

a. forming a fluorescent film assembly by applying a layer of a fluorescent material in a binder of polyvinyl alcohol to a support consisting of a polystyrene paper:

b. converting the polyvinyl alcohol binder to a water insoluble substance by treating the layer with formaldehyde;

c. bonding the fluorescent film assembly onto the surface of the substrate by means of a water-soluble, organic solvent insoluble binder, the fluorescent layer being bonded to the surface of said substrate;

d. dissolving said polystyrene paper from the resulting article by applying an organic solvent to said article; and

e. drying said organic solvent containing said polystyrene dissolved therein, whereby a thin coating of polystyrene is formed on the layer containing the fluorescent substance.

2. A method of forming a fluorescent screen according to claim 1, wherein said method includes the steps of preparing a fluorescent ink by mixing a fluorescent substance with polyvinyl alcohol, applying said ink onto said one side of the polystyrene paper, and irradiating said applied ink so as to render said ink water-insoluble.

3. A method of forming a fluorescent screen according to claim 1, wherein said method includes the steps of forming a light-sensitive layer containing at least one fluorescent sub- I i V i I 

2. A method of forming a fluorescent screen according to claim 1, wherein said method includes the steps of preparing a fluorescent ink by mixing a fluorescent substance with polyvinyl alcohol, applying said ink onto said one side of the polystyrene paper, and irradiating said applied ink so as to render said ink water-insoluble.
 3. A method of forming a fluorescent screen according to claim 1, wherein said method includes the steps of forming a light-sensitive layer containing at least one fluorescent substance on said polystyrene paper, exposing said light-sensitive layer to light rays through a negative mask having a desired figure or pattern, and developing said light-sensitive layer by removing the unexposed areas thereof by water so as to leave the developed layer having the desired figure or pattern of fluorescent material, said remaining fluorescent material being subsequently transferred by bonding to the surface of the said substrate in said desired figure or pattern.
 4. The method of forming a fluorescent screen according to claim 1, wherein said substrate is a face plate of a cathode ray tube. 